[FEA] Support `polars.Expr.str.encode` in `cudf-polars`

**Is your feature request related to a problem? Please describe.**
Today on `branch-25.08`, we lack support for [polars.Expr.str.encode](https://docs.pola.rs/api/python/stable/reference/expressions/api/polars.Expr.str.encode.html). Running the example from the docs yields
```
import polars as pl
engine = pl.GPUEngine(raise_on_fail=True)

df = pl.DataFrame({"strings": ["foo", "bar", None]}).lazy()
df = df.with_columns(strings_hex=pl.col("strings").str.encode("hex"))
res = df.collect(engine=engine)
print(res)
```

```
NotImplementedError: String function '
```

**Describe the solution you'd like**
I'd like the above code to be able to execute using the polars GPU backend. There's not a native `libcudf` API I can find that converts a general string to `hex` or `base64`, the options supported by the polars CPU API.

**Describe alternatives you've considered**
N/A

**Additional context**
https://github.com/rapidsai/cudf/issues/16480